Potential Impact on Standards for Toxic Air Contaminants

Abstract

Since the Bhopal incident, the public has placed pressure on regulatory agencies to set community exposure limits for the dozens of chemicals that may be released by manufacturing facilities. More or less objective limits can be established for the vast majority of these chemicals through the use of risk assessment. However, each step of the risk assessment process (i.e., hazard identification, dose-response assessment, exposure assessment, and risk characterization) contains a number of pitfalls that scientists need to avoid to ensure that valid limits are established. For example, in the hazard identification step there has been little discrimination among animal carcinogens with respect to mechanism of action or the epidemiology experience. In the dose-response portion, rarely is the range of "plausible" estimated risks presented. Physiologically based pharmacokinetic (PB-PK) models should be used to understand the difference between the tissue doses and the administered dose, as well as the difference in target tissue concentrations of the toxicant between rodents and humans. Biologically-based models like the Moolgavkar-Knudson-Venzon (MKV) should be developed and used, when appropriate. The exposure assessment step can be significantly improved by using more sensitive and specific sampling and analytical methods, more accurate exposure parameters, and computer models that can account for complex environmental factors. Whenever possible, model predictions of exposure and uptake should be validated by biological monitoring of exposed persons (urine, blood, adipose) or by field measurements of plants, soil, fish, air, or water. In each portion of an assessment, the weight of evidence approach should be used to identify the most defensible value. In the risk characterization, the best estimate of the potential risk as well as the highest plausible risk should be presented. Future assessments would be much improved if quantitative uncertainty analyses were conducted. Procedures are currently available for making future assessments. By correcting some of these shortcomings in how health risk assessments have been conducted, scientists and risk managers should be better able to identify scientifically appropriate ambient air standards and emission limits.

Inhalation Transfer Factors for Air Pollution Health Risk Assessment

“…For an episodic pollutant release, the ITF is defined as (1) where C(t) is the incremental breathing-zone concentration of the pollutant caused solely by the emission source (g/m 3 ), Q B (t) is the breathing rate (m 3 /hr), and E(t) is the emission rate of the pollutant from the source (g/hr). For a release of short duration, the integrals would be evaluated for a sufficiently long time, t, to encompass the entire event.…”

“…1 Considerable attention has also been given to health risk evaluations for indoor pollution sources, such as environmental tobacco smoke. 2 Health risk assessments that focus on the impact of a single source or source category involve several steps.…”

Looking back at nuclear weapons facilities

“…Those epidemiologic studies generating the most interest suggest that clusters of cancer cases among nuclear weapons facility employees or the neighboring public may exist or could appear (7,8). The publication of studies indicating the existence of a cancer cluster frequently stimulates subsequent studies that point out methodological problems in-and refute the results of-the first studies (9)(10)(11)(12)(13)(14). This process leaves a confused and angry public that must be addressed by health officials.…”

Comments on Dr. Finkel's Paper on Solution Focused Risk Assessment (SFRA)

“…Having been involved in many of the major risk assessment debates over the past 30 years (Paustenbach et al 1990), I do believe too much effort has been expended at trying to precisely identify the likely best estimate of the exposure risk at particular doses of chemicals, rather than simply placing the various risks into one of three boxes and then making decisions about what actions to take. To be specific, proposals that embrace placing all of the various risk into category I (clearly worthy of continual reductions in exposure), category II (possibly worthy of attention but not necessarily immediate), and category III (based on current knowledge, the risk appears de minimus) have increasing appeal.…”